A detainee walks through the recreation yard at the detention center for 'enemy combatants' in Guantánamo Bay, Cuba. Photograph: John Moore / Getty Images

Today, the Senate Intelligence Committee will finally vote on whether to make public a disputed report on years of torture conducted by the Central Intelligence Agency. They should vote to declassify this landmark document, as much for what it will remind us as that which we can never know.

In two joint United Nations reports on the American prison at Guantánamo Bay and the intelligence community's secret detention facilities, I closely investigated the dark arts perpetrated during those dark years. The US government has always denied our findings, and the public prefers to believe the US government over the UN, even while George W Bush and Dick Cheney largely exaggerated the impact of torture in order to justify it.

It is about time we knew that Bush-era torture didn't work as well as we were told. But it is also time we had a more rational discourse on the practice and "legitimacy" of torture.

The debate should have been settled long before the dawn of sophisticated terror attacks, long before President Obama said "waterboarding was torture, and it was a mistake". The deliberate infliction of severe pain or suffering – legal or mental, on a powerless person, for the purpose of extracting a confession – was legal during the Middle Ages, and it was abolished from criminal procedure codes during the age of enlightenment and rationalism. Apart from obvious humanitarian and human rights concerns, the main argument for abolition, then, was a growing awareness: torture was an absolutely unreliable tool to receive confessions from criminal suspects that could actually be used in court.

Torture is still absolutely unreliable. It may occasionally provide "useful" information – it may "work" in the eyes of the torturer – but it never works as a method to extract a confession.

I have seen both sides: there are criminal offenders who, under the threat or actual application of torture, sign a true confession they never would have otherwise. But I have seen innumerable victims who were innocent, who had signed a false confession out of fear of torture – or because they were not strong enough to endure the pain.

The point is still absolutely simple: we do not know whether any such confession made under "enhanced" conditions is true or false. We can never know, in that room, whether a victim committed the crime or not.

If torture is used to extract information about facts, the veracity of which can easily be verified, it may, however, sometimes yield useful results. Take the case of Magnus Gäfgen, who kidnapped the 11-year-old son of a rich banker in Germany, for ransom, in September 2002. Gäfgen immediately confessed to the crime, but he wouldn't say where the boy was. With the threat of torture from the Frankfurt police, he did. But the boy was already dead. Nevertheless, the mere threat of physical violence could have saved the life of this innocent child.

It is the classic ticking time-bomb scenario: in exceptional cases, when the torture of one person might save many innocent lives, torture, we were told by the Bush Administration, is somehow the lesser evil and should, therefore, be permitted. If the favorite scenario of action movies – and, yes, some academics – becomes the main argument against torture, the torturers themselves only ever have to prove that torture "works", in some real or hypothetical cases, in order to legitimize its application, forever.

But this utilitarian argument simply misses the point. Torture is one of the most brutal human rights violations. Torture constitutes a direct attack on the very core of human dignity. This is not about reliability; this is about humanity.

This is why the international community has outlawed torture, absolutely and under all circumstances, as a reaction to the atrocities of the Nazis. And this is why the main message of the Senate report should not be over-estimated: we already know about the gruesome methods of torture and the black sites, from which I'm sure the CIA received intelligence it considered useful. But it's not that important whether torture yielded results that were useful, even if the report finds that the waterboarding of Khalid Sheikh Mohammed 183 times provided intelligence that "wasn't critical" – that the suspected al-Qaida member Abu Zubaida gave up information while he was in the hospital, not while he was waterboarded 83 times himself. It is important that torture is illegal, amoral and beyond utilitarian logic.

Sometimes torture works. But this may never be used as an argument to justify torture, because torture is, with very good reasons and on the basis of historical experience, absolutely prohibited. It should remain that way, forever.